The invention relates to a controller for controlling a motor, and particularly to a controller for controlling an induction motor, and still more particularly to a controller for controlling an induction motor in a fluid pump.
It is known to provide an air handling system such as a heating, ventilating or air conditioning ("HVAC") system with a blower or fluid pump that either pushes air over or draws air across a heat exchanger or cooling coil to heat or cool the air, respectively, and transfer the air through a system of ducts and vents to a room or rooms where a thermostat is located. The thermostat provides feedback to the system to indicate the temperature in the room or rooms. In this way, the temperature of the air in those rooms is controlled. The blower includes a motor and the HVAC system usually also includes a controller for controlling the motor in response to various parameters such as room air temperature, air flow rate, motor speed, and motor torque.
It is also known that the efficiency of the heat transfer between the air and the heat exchanger or cooling coil is directly dependent upon the flow rate of air across the heat exchanger or the cooling coil. Moreover, it is known that the efficiency of the heat transfer process can be maximized by maintaining the flow rate at a specific set point. The set point or flow rate at which the heat transfer is most efficient is determined empirically (typically by the manufacturer of the HVAC system), and is programmed into the thermostat of the system. As vents in the system are opened or closed, however, the load on the motor changes, thereby changing the motor speed, blower output and stator current. The changing loads experienced by the motor make precise control of the blower output extremely difficult.
Many techniques for controlling the air flow rate of a blower in an HVAC system have been developed and have consistently revolved around the use of a permanent magnet brushless d.c. motor. Permanent magnet brushless d.c. motors have been used because they are relatively easy to control and provide good performance in low power air handling applications. However, permanent magnet brushless d.c. motors are typically more expensive and less rugged than other types of motors such as induction motors. One known method of controlling a permanent magnet brushless d.c. motor is shown and described in U.S. Pat. No. 4,978,896.